1. Field of the Invention
The invention relates to a heating apparatus for the conductive resistance heating of melts, in particular for the rapid melting-down of batch and/or for the refining and/or conditioning of melts, which comprises at least one electrode for the conductive heating of melts, in particular a cooled electrode for the conductive heating of melts.
2. Description of Related Art
The conductive heating of melts is used, inter alia, in the refining of glass melts. During refining, bubbles are expelled from the glass melt, preferably by adding special refining agents. To enable the bubbles to be expelled, it is desirable for the viscosity of the melt to be as low as possible. This is generally achieved by high temperatures being set in a refining zone of the melt. However, it is impossible to select any desired high temperatures in the melt, since the temperature which can be set is subject to restrictions in terms of the finite thermal stability of the components of the melting apparatus. In particular, there are no known contact materials which can be used in practice and have a long-term stability at temperatures above 1 700° C.
In order nevertheless to allow higher temperatures to be reached, it is known to use water-cooled, metallic wall parts for the melting tank. However, the cooled wall parts cause high energy losses, which have to be compensated for by heating again. Heating which is suitable for high-temperature refining therefore has to apply sufficiently high heating powers.
Radiofrequency heating, inter alia, is used to heat the melt. This technique is also used in particular together with the skull technique, i.e. with cooled wall parts. However, radiofrequency heating is not suitable for all types of glass, since the melts have to have a certain minimum conductivity. For example, the method is unsuitable for melt conductivities lower than 0.01 Ω−1 cm−1. However, the process stability of radiofrequency heating becomes so poor even at conductivities below approximately 0.1 Ω−1 cm−1 that this technique is of only limited use or is scarcely viable in these cases.
Alkali metal ions are particularly responsible for achieving a good conductivity in glass melts. On the other hand, glasses which contain little if any alkali metal often have only a poor conductivity. However, in particular glasses of this type are often special technical-grade glasses, in which a particularly high purity and absence of bubbles is important, yet these glasses can only be treated with difficulty by radiofrequency heating, on account of their poor conductivity.
In addition to radiofrequency heating, it is also possible to use water-cooled electrodes for the conductive heating of melts. GB 644 463 describes a hollow electrode which is cylindrical in form or in some other form, can be made from metal and is provided in the interior with a coaxially arranged hollow body within which a cooling fluid circulates. GB 644,463 has furthermore disclosed a water-cooled platinum electrode which can only be operated effectively at substantially one thermal operating point. In this case, it must always be ensured that the platinum electrode is not damaged by overheating. However, since heating of the melt also presents a risk of damage to the platinum electrode, the coolant supplied will generally effect cooling which maintains a safe thermal distance below the temperatures at which the platinum can be damaged. However, as a result some of the heating power of the conductive electrodes is “cooled away” again and has to be compensated for by an increased supply of energy. The additional heating power is absorbed again by an increased cooling power, and the thermal safety margin represents an extremely unsatisfactory overall energy balance when using conventional cooled electrodes.
Furthermore, in the event of uncontrollable and excessive cooling of the conventional electrodes, there is a risk of the conductive heating being impossible to start, on account of a high transfer resistance between electrode and melt, caused by glass frozen onto the electrode.
DE 199 25 554 A1 describes a bottom electrode for metallurgical melting vessels, in which a cooling plate is arranged at a distance below the carrier plate which carries the contact elements. A cooling medium flows through the cooling space which is thereby formed; the cooling action of this cooling medium is intensified by cooling fins which are arranged in the cooling space and are welded to the carrier plate and preferably also to the cooling plate.
Furthermore, the local cooling power on the surface of these electrodes is substantially predetermined by the arrangement of the cooling passages and by the electrode geometry, and therefore cannot be adapted to the external conditions.